1. Field of the Invention
This invention relates to an electrophotographic photosensitive member and an electrophotographic apparatus making use of the same, and more particularly relates to an electrophotographic photosensitive member most suited for printers, facsimile machines, copying machines and so forth in which light of 380 nm or more to 500 nm or less in wavelength is used in exposure, and an electrophotographic apparatus making use of the same.
2. Related Background Art
In electrophotographic apparatus used as printers, facsimile machines, copying machines and so forth, a photosensitive member charged electrostatically by a charging member is irradiated with light and the areas other than those corresponding to images or the areas corresponding to images are exposed to light to form, on the photosensitive member, electrostatic latent images corresponding to the images. Then, a toner is fed thereto to develop the electrostatic latent images, and the toner having adhered to the electrostatic latent images is transferred to a transfer material and thereafter fixed thereto. Meanwhile, the surface of the photosensitive member from which toner images have been transferred to the transfer material is de-charged. Through these steps, the formation of images is performed.
Photoconductive materials in photosensitive members used in the image formation of such electrophotographic apparatus are required to have properties such that they are highly sensitive, have a high SN ratio [light current (Ip)/dark current (Id)], have absorption spectra adapted to spectral characteristics of electromagnetic waves to be applied, have a high response to light, have the desired dark resistivity and are harmless to human bodies when used. In particular, in the case of photosensitive members set in electrophotographic apparatus used as business machines in offices, the harmlessness in their use is important. As a photoconductive material showing such superior properties, amorphous silicon (hereinafter also simply “a-Si”) is available, and is widely used as a light-receiving material of the electrophotographic photosensitive member.
In such photosensitive members making use of amorphous silicon, it is common to form a photoconductive layer composed of a-Si, on a conductive substrate heated to 50° C. to 350° C., by a film forming process such as vacuum deposition, sputtering, ion plating, heat-assisted CVD, light-assisted CVD or plasma-assisted CVD. In particular, the plasma-assisted CVD is preferably employed in which source gases are decomposed by high-frequency or microwave glow discharging to form a-Si deposited films on the substrate. On the photoconductive layer thus formed, a surface layer which affords durability to wear and to service environments such as temperature and humidity is superposed to produce a photosensitive member suited for practical use.
In order to achieve improvements in photoconductive properties such as dark resistivity, photosensitivity and response to light and service environmental properties such as humidity resistance, and further in stability over time, running (extensive operation) performance and so forth of photoconductive members having the photoconductive layer constituted of such an amorphous-silicon deposited film, a photosensitive member is proposed in which an a-Si:N photosensitive member comprising a substrate, a barrier layer, a photoconductive layer and a surface layer is produced from SiH4, H2, N2 and B2H6 and is so constituted as to be in a bias state reverse to p-i-n junction, by specifying source gas flow rates for the respective layers, as disclosed in, e.g., Japanese Patent Application Laid-open No. H05-150532. Japanese Patent Application Laid-open No. H08-171220 also discloses an electrophotographic photosensitive member having on a conductive substrate a photoconductive layer formed of a-Si and a surface layer formed of amorphous silicon nitride, wherein in the outermost surface of the photosensitive member, the element compositional ratio of N/Si ranges from 0.8 to 1.33 and the element compositional ratio of O/Si ranges from 0 to 0.9.
In addition, methods for electrostatically charging such an a-Si photosensitive member include a corona charging method, which makes use of corona charging; a roller charging method, which makes use of a charging roller and performs charging by direct discharging; and an injection charging method, which secures contact area sufficiently by using magnetic particles or the like and provides electric charges directly to the photosensitive member surface to perform charging. In particular, the corona charging method and the roller charging method make use of discharging, and hence discharge products tend to adhere to the photosensitive member surface. In addition, the a-Si photosensitive member has a surface layer having much higher hardness than an organic photosensitive member, and hence the discharge products tend to remain on the surface, where the discharge products and moisture may combine because of adsorption of moisture in an environment of high humidity or the like. This may cause the surface to have a low resistivity to easily move surface electric charges and result in a phenomenon of image deletion. Hence, various measures have been required to be employed in some cases on how to rub the surface and how to perform temperature control.
On the other hand, the injection charging method is a charging method in which electric charges are directly provided from the part having come into contact with the photosensitive member surface without using discharging, and hence the phenomenon called image deletion can not easily take place.
In addition, the injection charging method, which is a contact charging method, is of a voltage control type, while the corona charging method is of a current control type. Hence, the former has such an advantage that any non-uniformity of charging potential can be relatively easily reduced.
In conventional a-Si photosensitive members, the improvements of properties have each individually been achieved in respect of electrical, optical and photoconductive properties such as dark resistance values, photosensitivity and photoresponsiveness, and further in respect of durability over time and running performance. In the actual circumstances, however, there is room for further improvement in order to achieve overall improvement in properties.
In particular, in recent years, the transition to digital processing and color image formation has rapidly advanced, and electrophotographic apparatus are more highly required than ever to achieve high image quality. The high image quality herein termed refers to being high-resolution, being high definition, being free of density non-uniformity, and being free of image defects (such as blank areas or voids and black dots). Besides, the electrophotographic apparatus are also rapidly more required to have more high-speed and higher running performance, and, in electrophotographic photosensitive members, it is sought to improve electrical properties and photoconductive properties, to improve uniformity and reduce image defects, and also to greatly enhance performances including running performance and environmental resistance (adaptability to changes in temperature and humidity).
For example, in order to enhance the resolution of images, it is effective for toners to have a small particle diameter and also for laser beams for image formation to have a small spot diameter. Means for reducing the spot diameter of a laser beam include improving the precision of an optical system through which laser beams are applied to the photoconductive layer, and increasing the aperture ratio of an image forming lense. In order to increase the aperture ratio of an image forming lense, the lense must be made large and mechanical precision must be improved. For such reasons, it is difficult to avoid enlarging an apparatus and causing a rise in cost.
Accordingly, in recent years, research is focused on a technique in which laser beams are made to have a short wavelength to reduce the spot diameter small and to enhance the resolution of electrostatic latent images. This relies on the fact that the minimum value of the spot diameter of a laser beam is in direct proportion to the wavelength of the laser beam. In conventional electrophotographic apparatus, laser beams having an oscillation wavelength of 600 to 800 nm have been commonly used in performing image exposure. Making this wavelength shorter can make the resolution of images higher. In recent years, the development of semiconductor lasers having short oscillation wavelengths has rapidly advanced, and semiconductor lasers having an oscillation wavelength of about 400 nm have been put into practical use. Thus, it has been sought to provide a photosensitive member adaptable to the light of such a short wavelength band.
As for a measure taken when such short-wavelength light is used, Japanese Patent Application Laid-open No. 2000-258938 discloses an image forming apparatus characterized in that a photosensitive layer is a layer which contains a-Si hydride and that an exposure means has an ultraviolet bluish-purple laser beam oscillator having a chief oscillation wavelength of 380 nm to 450 nm. Japanese Patent Application Laid-open No. 2002-311693 discloses an electrophotographic apparatus characterized in that an a-Si type photosensitive member is used and that an electric field applied to the photosensitive member at the time point of exposure to image forming light beams is 150 kV/cm or more and the image forming light beams have a wavelength of 500 nm or less.
Where a semiconductor laser having an oscillation wavelength of about 400 nm is used in imagewise exposure, photosensitive members are sought firstly to have sufficient sensitivity to exposure wavelength light, and secondly to absorb almost no exposure wavelength light in the surface layer. The a-Si type photosensitive layer has a peak sensitivity at about 600 to 700 nm, and hence it has a sensitivity at about 400 to 410 nm, though being a little inferior to the peak sensitivity, if conditions are skillfully designed. For example, it is usable even when a 405 nm short-wavelength laser is used. Nevertheless, in respect of sensitivity, it may have a half sensitivity or thereabout compared with the peak sensitivity. In such a case, it is therefore preferable that almost no absorption takes place at the surface layer. However, in the cases of amorphous silicon carbide (hereinafter “a-SiC”) type materials and amorphous carbon (hereinafter “a-C”) type materials both having conventionally preferably been used in the surface layer, there has been a tendency for the absorption to take place greatly at about 400 to 410 nm. More specifically, when the a-SiC type materials are used, it has been possible for the surface layer to be improved in transmittance by skillfully designing conditions and also to be managed by setting its layer thickness to be small to a certain degree. However, the surface layer is fated to be gradually scraped on by friction in the copying machine, and hence it is required to have a certain or more layer thickness in order to make full use of the property the a-SiC type materials have, i.e., the long lifetime. Thus, it has come about in some cases that the absorption and lifetime in the surface region fall into the relation of a trade-off. Also, in the case of the a-C type materials, it has been possible to form a film having a good transmittance, depending on conditions. In such a case, however, the film may have a structure close to a polymer to have a low hardness and have a too high resistance. Thus, in the case of the a-C type materials, it has come about in some cases that the transmittance and the hardness or the resistivity are incompatible with each other.
In contrast with these materials, where amorphous silicon nitride (hereinafter “a-SiN”) type materials are used, it has been known that the coefficient of absorption at about 400 to 410 nm can be lowered by optimizing conditions. However, such a film is difficult to use as the surface layer of the photosensitive member, and has never been put into practical use.
Conditions for forming an a-SiN type film suitable for the surface layer are disclosed also in Japanese Patent Application Laid-open No. H05-150532 (Patent Document 1). However, even in this case, as the wavelength of light used in exposure, up to 550 nm is only taken into account. Moreover, even at such an exposure wavelength of 550 nm, a low sensitivity results when the surface layer has a layer thickness of more than 0.8 μm.